Hitherto, magnetic heads with electromagnetic induction being an operating principle (induction type magnetic heads) have been used and diffused. However, in using them in a higher density recording/reproduction region, limits initiate to be seen. Then, reproducing heads with magnetic resistance (MR) being an operating principle are proposed and initiate to be used in hard discs, etc. The MR heads provide a reproducing output of several times higher than the induction type magnetic heads. Further, since the MR heads do not use an induction coil, they lower an instrumental noise such as impedance noise and reduce a noise of the magnetic recording media, thereby enabling to obtain a high S/N ratio. In conventionally used coating type magnetic recording media that have excellent productivity and can be provided at a low cost, magnetic recording layers suited for MR heads are being investigated. Magnetic recording media comprising a nonmagnetic support having thereon a substantially nonmagnetic lower layer and a magnetic layer having a ferromagnetic fine powder dispersed in a binder in that order are able to make the magnetic layer thin while keeping the surface smoothness and hence, are frequently applied at present in high density coating type magnetic recording media (see JP-B-4-71244).
In order to realize high density recording, it is important to make the particle size of the magnetic powder smaller. However, for example, in developing coating type magnetic recording media having an areal recording density exceeding 0.3 Gbit/inch2, there has been especially revealed a problem such that the smaller the particle size of magnetic particles, the larger the noise is. When the particle size of magnetic powders is made small, it becomes difficult to disperse the magnetic powders in a binder during preparation of a coating material for magnetic layer, and it becomes impossible to keep the surface smoothness of the magnetic layer. As a result, it becomes difficult to obtain desired low noise media. For suppressing the noise, it is necessary that (1) coagulation of magnetic materials be dissolved and (2) the surface of the magnetic layer be made smooth.
Magnetic recording tapes are required to have good running durability in addition to excellent electromagnetic conversion characteristics. For achieving this, there are made attempts to improve the running durability by, for example, providing a back coating layer and proving protrusions on the base surface or adding coarse particulate carbon having a particle size of 0.1 μm or more to the back coating layer to roughen the surface. In the case where the surface of a back coating layer is roughened by such a method, there was a defect of causing so-called “reverse taking” such that when a magnetic recording tape is wound up around a hub and stored or treated, the back coating layer comes into press contact with a magnetic layer, whereby unevennesses of the back coating layer are taken into the magnetic layer, resulting in a reduction of the electromagnetic conversion characteristics. For dissolving such a “reverse taking” problem, there is made an attempt to smoothen the surface of the back coating layer.
For example, JP-A-10-116414 proposes to use a back coating layer containing carbon black as primary particles or aggregates having a diameter larger than the long axis length of a metal magnetic powder and having a surface smoothness of not more than 0.01 μm in terms of center line average roughness. In this case, it is possible to suppress an increase in noise during winding and storage. However, such an effect is not useful in the case where reverse taking has already occurred at the time of preparation of the tape. Further, this patent document does not describe at all durability of the back coating layer. When the surface of the back coating layer is smoothened, a friction coefficient against guides of reproducing units such as VTR increases, leading to reduction of running stability.
The presence state of a lubricant on the medium surface greatly contributes to the running durability. For example, JP-A-2001-222811, JP-A-2001-229521 and JP-A-2001-325716 propose to improve the running property by setting up an amount of a lubricant on the surface and surface hardness in the magnetic layer side within specific ranges. However, in these patent documents, such proposals are limited to the magnetic layer regardless of magnetic disc-like media or tape media. The patent documents do not describe durability of back coating layer at all.
JP-A-11-259851 proposes a back coating layer of a high adhesion strength, containing particulate oxides of titanium oxide, α-iron oxide or a mixture thereof and carbon black and further from 10 to 40 parts by weight, based on 100 parts by weight of the total weight of the particulate oxides and carbon black, of a binder. In this case, the patent document describes that it is possible to improve the output and keep the running durability. However, the whole of the system as disclosed is of a system of induction type magnetic head that is relatively insensitive to noise, and the patent document does not describe influence against the noise in the magnetic layer side. Accordingly, we cannot stop saying that improvements are necessary as a coating type magnetic recording medium having an areal recording density exceeding 0.3 Gbit/inch2.
On the other hand, a demand of realization of thin layers extends over not only magnetic layers but also the whole of layers constituting a magnetic recording tape. In recent years, media for computer storage are required to have high capacity and make the thickness of the whole of a tape thin. For making the thickness of magnetic recording tapes thin, there have hitherto been employed a method of making a flexible support thin and a method of making a nonmagnetic layer between a flexible support and a magnetic layer of a coating type magnetic recording tape thin. However, there were involved defects such that when the flexible support is made thin to less than a certain range, the running durability is lowered and that when the nonmagnetic layer is made thin, reduction in output, increase in error rate, or increase in drop-out likely occurs. A back coating layer cannot be omitted for the purpose of keeping the running property good. Further, when the back coating layer is made thin, since adhesion strength between a nonmagnetic layer and a support is lowered, the back coating layer likely peels away during repeated running, or protrusions on the surface of the back coating layer cause reverse taking into a magnetic layer, leading to reduction in output.
As described above, the conventional technologies have not yet provided magnetic recording tapes that are sufficiently good in both electromagnetic conversion characteristics and running durability. Especially, in these days when the thickness of the whole of magnetic recording tape is required to be made thin, nevertheless magnetic recording tapes containing a thin back coating layer and having good electromagnetic conversion characteristics and running durability are required, satisfactory magnetic recording tapes have not been provided yet.